An electrostatic sensor is used to measure electrostatic charge, primarily by measuring the effect, on an electrode, of lines of flux emanating from the charge being measured. The sensing electrode is associated with an earthed shield and provides an output which depends on the magnitude of the charge being sensed. More particularly, the sensor output will usually be proportional to the magnitude of the charge in front of the sensing surface of the electrode, measured as a weighted average across the area of the surface. The degree of sensitivity is also related to the area of the sensing surface and the proximity of the earth shield. The field of view to which the sensor responds can be modified, for example by modifying the shape of the earth shield or by moving the sensor toward or away from the object of interest. A measurement made by the sensor at a particular point in time will represent the view of the sensor at that point in time. In the case of moving objects, a sequence of such measurements can be used to provide a more complete understanding of the system being observed, but this approach can give rise to problems, particularly with a fast moving or quickly changing system.